Pwm control circuit and pwm control method

ABSTRACT

The PWM control circuit includes a polarity determination unit, a full wave rectification unit, an adjustment unit that generates an adjusted waveform signal by adjusting waveform of the full wave rectification signal, and a carrier signal generating unit that generates a fixed frequency carrier signal. The PWM control circuit further includes a comparator that generates an original PWM signal by comparing the adjusted waveform signal and the carrier signal, and a PWM waveform shaping unit that generates a first PWM signal for the positive polarity section and a second PWM signal for the negative polarity section, by shaping the original PWM signal according to the polarity signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The is a continuation application of U.S. Ser. No. 12/257,817 filed Oct.24, 2008, which claims the priority based on Japanese Patent ApplicationNo. 2007-286447 filed on Nov. 2, 2007, all of which are herebyincorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a PWM control circuit used in motorsand the like.

2. Description of the Related Art

A PWM control circuit is described, for example, in JP2002-84772A.

With conventional PWM control circuits for motor control, PWM signalsare generated by comparing a sine wave signal generated from the motorsensor output and a triangular signal as a reference signal. However,there has been demand for a PWM control circuit that would furtherincrease the motor efficiency. This kind of demand is not limited to PWMcontrol circuits for motor control, but is also an issue common togeneral PWM control circuits.

SUMMARY OF THE INVENTION

An object of the invention is to provide technology that makes possiblePWM control with better efficiency.

According to an aspect of the present invention, there is provided a PWMcontrol circuit that generates PWM signals based on an analog sensoroutput from a sensor provided in a device to be controlled. The PWMcontrol circuit includes a polarity determination unit that judgespositive polarity sections and negative polarity sections of the analogsensor output to generate a polarity signal, a full wave rectificationunit that generates a full wave rectification signal by doing fullrectification of the analog sensor output, an adjustment unit thatgenerates an adjusted waveform signal by adjusting waveform of the fullwave rectification signal, and a carrier signal generating unit thatgenerates a fixed frequency carrier signal. The PWM control circuitfurther includes a comparator that generates an original PWM signal bycomparing the adjusted waveform signal and the carrier signal, and a PWMwaveform shaping unit that generates a first PWM signal for the positivepolarity section and a second PWM signal for the negative polaritysection, by shaping the original PWM signal according to the polaritysignal.

With this PWM control circuit, the full wave rectification signal isgenerated from the analog sensor output, and the adjusted waveformsignal is generated by adjusting this full wave rectification signal, soit is possible to obtain a signal with a desirable waveform. Also, thePWM signal is generated using this adjusted waveform signal, so moreefficient PWM control is possible.

Note that the present invention can be realized with various modes, forexample, it can be realized with modes such as a PWM control circuit andmethod, an electric motor and the control method thereof, or anactuator, device, portable device, electronic device, mobile body, robotor the like that use these.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the circuit configuration of thebrushless motor of this embodiment;

FIGS. 2A-2C show the positional relationship of a magnet array and acoil array with the motor main unit, and the relationship of themagnetic sensor output and the coil back electromotive force waveform;

FIG. 3 is a block diagram showing the internal constitution of theanalog waveform adjustment unit and the DA converter;

FIGS. 4A-4F are timing charts showing the waveform of the input/outputsignal of each unit of the analog waveform adjustment unit;

FIGS. 5A and 5B shows the internal constitution and operation of theexcitation interval setting unit;

FIGS. 6A and 6B are block diagram showing an example of the internalconstitution of the PWM control unit;

FIG. 7 is a block diagram showing the internal constitution of the fullbridge circuit;

FIGS. 8A and 8B show an example of the pulse width adjustment with thedrive control circuit of this embodiment;

FIGS. 9A and 9B show another example of the pulse width adjustment withthe drive control circuit of this embodiment;

FIG. 10 illustrates a projector utilizing a motor according to a mode ofthe present invention;

FIGS. 11A-11C illustrate a mobile phone of fuel cell type utilizing amotor according to a mode of the present invention;

FIG. 12 illustrates an electric bicycle (power-assisted bicycle) as anexample of a moving vehicle utilizing a motor/generator according to amode of the present invention; and

FIG. 13 illustrates an example of a robot utilizing a motor according toa mode of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Next, we will describe modes of implementing the present invention inthe following sequence.

A. Embodiment

B. Variation Example

A. Embodiment

FIG. 1 is a block diagram showing the circuit configuration of thebrushless motor of the present embodiment. This brushless motor isequipped with a motor main unit 100 and a drive control circuit 200. Themotor main unit 100 has an electromagnetic coil 110 and a magneticsensor 120. The drive control circuit 200 is equipped with a PWM controlunit 210, a full bridge circuit 220, a polarity determination unit 230,an analog waveform adjustment unit 240, a DA converter 250, and a CPU260. The polarity determination unit 230 determines the positivepolarity sections and the negative polarity sections of the analogoutput SSA of the magnetic sensor 120, and generates a polarity signalSp. The analog waveform adjustment unit 240 adjusts the waveform of theanalog sensor output SSA and generates an adjusted waveform signalSwave. The DA converter 250 supplies various types of setting valuesused for waveform adjustment to the analog waveform adjustment unit 240.Note that these setting values are values directed to the DA converter250 from the CPU 260. The PWM control unit 210 executes PWM controlbased on the waveform signal Swave and the polarity signal Sp, andgenerates first and second PWM signals DRVA1 and DRVA2. Note that thesesignals DRVA1 and DRVA2 are also called “drive signals.” The full bridgecircuit 220 supplies drive voltage to the electromagnetic coil 110according to the drive signals DRVA1 and DRVA2. The internalconstitution of the circuits 210 to 250 will be described later. Withthe description hereafter, as can be seen from FIG. 1, we are describinga circuit for single phase drive using a single phase portion of theelectromagnetic coil 110 and the magnetic sensor 120, but it is alsopossible to easily realize this by using the same constitution for eachphase for a multi phase drive circuit of two phases or more. In thiscase, a circuit configuration having the electromagnetic coil 110constitution and the magnetic sensor 120 constitution according to thephase count is used.

FIGS. 2A-2C show the positional relationship of a magnet array and acoil array with the motor main unit 110, and the relationship of themagnetic sensor output and the coil back electromotive force waveform.Note that “back electromotive force” is also called “induced voltage.”As shown in FIG. 2A, the motor main unit has a stator unit 10 includinga plurality of coils 11 to 14, and a rotor unit 30 including a pluralityof magnets 31 to 34. The coils 11 to 14 correlate to the electromagneticcoil 110 in FIG. 1. The four magnets 31 to 34 are arranged at a fixedmagnetic pole pitch Pm, and magnets adjacent to each other aremagnetized in the reverse direction. Also, the coils 11 to 14 arearranged at a fixed pitch Pc, and coils adjacent to each other areexcited in the reverse direction. With this example, the magnetic polepitch Pm is equal to the coil pitch Pc, and with the electrical anglecorrelates to π. Note that the electrical angle 2π is correlated to themechanical angle or distance that the drive signal phase moves whenchanged by 2π. With this embodiment, when the phase of the drive signalchanges by 2π, the rotor unit 30 moves by twice the magnetic pole pitchPm.

Of the four coils 11 to 14, the first and third coils 11 and 13 aredriven by drive signals of the same phase, and the second and fourthcoils 12 and 14 are driven by drive signals for which the phase isdisplaced by 180 degrees (=π) from the drive signals of the first andthird coils 11 and 13. With normal two phase drive, the phase of thedrive signals of the two phases (A phase and B phase) are displaced by90 degrees (=π/2), and there is no case of the phase displacement being180 degrees (=π). Also, with the motor drive method, there are manycases for which two drive signals for which the phase is displayed by180 degrees (=π) are regarded as being the same phase. Therefore, thedrive method for the motor of this embodiment can be thought of as beinga single phase drive.

FIG. 2A shows the positional relationship of the magnets 31 to 34 andthe coils 11 to 14 when the motor is stopped. With the motor of thisembodiment, the magnetic yoke 20 at each coil 11 to 14 is provided at aposition displaced slightly in the normal rotation direction of therotor unit 30 from the center of each coil. Therefore, when the motor isstopped, the magnetic yoke 20 of each coil is attracted by the magnets31 to 34, and the rotor unit 30 stops at the position for which themagnetic yoke 20 faces the center of each magnet 31 to 34. As a result,the motor stops at the position for which the center of each coil 11 to14 is displaced from the center of each magnet 31 to 34. Also, at thistime, the magnetic sensor 120 is also at a position slightly displacedfrom the boundary of the adjacent magnet. The phase at this stopposition is α. The phase α is not zero, but it is preferable to be setto a small value close to zero (e.g. approximately 5 to 10 degrees), ora value close to π/2 (e.g. approximately 85 to 95 degrees).

FIG. 2B shows an example of the waveform of the back electromotive forcegenerated at the coil, and FIG. 2C shows an example of the outputwaveform of the magnetic sensor 120. The magnetic sensor 120 cangenerate analog sensor output SSA of almost the same shape as the backelectromotive force of the coil when the motor is operating. However,the output SSA of the magnetic sensor 120 shows a value that is not 0even when the motor is stopped (except when the phase is an integralmagnitude of π). Note that the back electromotive force of the coil hasa tendency to rise with the motor rotation speed, but the waveform shape(sine wave) is kept at almost the same shape. As the magnetic sensor120, for example, it is possible to use a Hall IC which uses the Halleffect. With this example, the sensor output SSA and the backelectromotive force Ec are both sine wave shapes or waveforms close to asine wave. As described later, the drive control circuit of this motoruses the sensor output SSA and applies to each coil 11 to 14 a voltageof almost the same waveform as the back electromotive force Ec.

FIG. 3 is a block diagram showing the internal constitution of theanalog waveform adjustment unit 240 and the DA converter 250. The analogwaveform adjustment unit 240 has an amplifier unit 241, a full waverectification unit 242, an offset adjustment unit 243, a gain adjustmentunit 244, and an excitation interval setting unit 245. The DA converter250 has an amplification factor setting unit 251 for setting anamplification factor Am, an offset setting unit 253 for setting anoffset value Os, a gain setting unit 254 for setting a gain value Ga,and a threshold value setting unit 255 for setting a threshold voltageVth. Each setting unit 251, 235-255 inside the DA converter 250 does DAconversion of each type of setting value given from the CPU 260 (FIG.1), and supplies the various setting values Am, Os, Ga, and Vth asanalog signals to each corresponding unit inside the analog waveformadjustment unit 240.

FIGS. 4A-4F are timing charts showing the waveform of the input/outputsignal of each unit of the analog waveform adjustment unit 240. Thesensor output SSA (FIG. 4A) has an almost symmetrical waveform with theground potential GND as the center. The sensor output SSA preferably hasa sine wave shape. The amplifier unit 241 generates thepost-amplification signal Sa (FIG. 4B) by amplifying this sensor outputSSA using the amplification factor Am. The full wave rectification unit242 generates a full wave rectification signal Sb (FIG. 4C) by doingfull wave rectification of this post-amplification signal Sa. The offsetadjustment unit 243 generates the signal Sc for which the referencelevel of the full wave rectification signal Sa is offset according tothe offset value Os, and the gain adjustment unit 244 generates thesignal Sd for which this signal Sc is amplified according to the gainvalue Os (FIGS. 4D and 4E). Note that FIG. 4D illustrates a rise in thereference level of the signal Sb by Vbt volts according to the offsetvalue Os. The excitation interval setting unit 245 generates a signalSwave for which only part of the signal Sd is valid and the other partis invalid according to the threshold voltage Vth (FIG. 4F). Theinternal constitution and operation of the excitation interval settingunit 245 will be described later.

Note that with the analog waveform adjustment unit 240, the reason foradjusting the offset and the gain is because there is a possibility ofthe waveform being distorted due to sensor attachment error or the likebecause the sensor output SSA does not necessarily have a desirablewaveform (a sine wave shape, for example). When the waveform of thesensor output SSA is distorted, by adjusting the offset and gain, it ispossible to come closer to a desirable waveform. Also, as a result, itis possible to increase the motor efficiency.

FIGS. 5A and 5B show the internal constitution and operation of theexcitation interval setting unit 245. This excitation interval settingunit 245 has an analog comparator 245 a and a buffer circuit 245 b. Theanalog comparator 245 a compares the signal Sd after the offset/gainadjustment and the threshold voltage Vth, to produce an enable signalEnb which goes to H level when Vth≦Sd (see FIG. 5B). This enable signalEnb is supplied to the enable terminal of the buffer circuit 245 b. Whenthe enable signal Enb is H level, voltage which is proportionate to thesignal Sd is output from the buffer circuit 245 b, and when the enablesignal Enb is L level, output from the buffer circuit 245 b stops. As aresult, as shown in FIG. 5B, the output signal Swave of the excitationinterval setting unit 245 becomes a signal for which only part of thesignal Sd is valid and the other part is invalid. Following, the outputsignal Swave of the excitation interval setting unit 245 is called the“adjusted waveform signal.”

As can be understood from FIGS. 4A-4F as well, the signal Sd after theoffset/gain adjustment has a waveform similar to the signal for whichthe sensor output SSA underwent full wave rectification, so the enablesignal Enb (FIG. 5B) has a cycle correlating to the half cycle of thesensor output SSA. Therefore, when the position at which the sensoroutput SSA polarity reverses is defined to be the π phase point, it canbe understood that the enable signal Enb makes the signal Sd valid inthe symmetrical valid section with the π/2 phase point as the center,and makes the signal Sd invalid in the symmetrical invalid section withthe π phase point as the center. This kind of excitation intervalsetting unit 245 has a function of improving the motor efficiency, andthis point will be described later.

Note that the combination of the offset adjustment unit 243, the gainadjustment unit 244, and the excitation interval setting unit 245functions as an adjustment unit that adjusts the waveform of the fullwave rectification signal Sb. The sequence of the gain adjustment andthe offset adjustment may be reversed. It is also possible to omit partof the units 241 to 245 in the analog waveform adjustment unit 240. Forexample, it is possible to omit circuits 241, and 243 to 245, other thanthe full wave rectification unit 242.

FIGS. 6A and 6B are block diagrams showing an example of the internalconstitution of the PWM control unit 210 (FIG. 1). With the exampleshown in FIG. 6 A, the PWM control unit 210 is equipped with a sawtoothwaveform generating unit 211, an analog comparator 212, and a PWMwaveform shaping circuit 213. The sawtooth waveform generating unit 211is a circuit for generating fixed cycle sawtooth wave signals Ssaw.However, the cycle of the sawtooth wave Ssaw may be changed asnecessary. The analog comparator 212 generates an original PWM signalSpwm by comparing this sawtooth wave signal Ssaw and the adjustedwaveform signal Swave supplied from the analog waveform adjustment unit240.

The PWM waveform shaping circuit 213 generates the first PWM signalDRVA1 and the second PWM signal DRVA2 based on this original PWM signalSpwm and on the polarity signal Sp given from the polarity determinationunit 230. Note that the polarity signal Sp, as described previously, isa signal that is H level in the positive polarity section for which thesensor output SSA is positive, and is L level in the negative polaritysection for which the sensor output SSA is negative. The PWM waveformshaping circuit 213 has two AND circuits 214 and 215 and an inverter(NOT circuit) 216. The first AND circuit 214 allows the original PWMsignal Spwm to pass through as is when the polarity signal Sp is Hlevel, and blocks passage of the original PWM signal Spwm when thepolarity signal Sp is L level, to thereby generate the first PWM signalDRVA1. A reverse signal of the polarity signal Sp is input to the secondAND circuit 215. Therefore, the second AND circuit 215 blocks thepassage of the original PWM signal Spwm when the polarity signal Sp is Hlevel, and also allows the original PWM signal Spwm to pass through asis when the polarity signal Sp is L level, to thereby generate thesecond PWM signal DRVA2.

Note that with the example in FIG. 6A, the signal DRVA1H is the drivesignal supplied to the upper arm transistor of the full bridge circuit220, and the signal DRVA1L is the drive signal supplied to the lower armtransistor of the full bridge circuit 220, but with the example in FIG.6A, these signals DRVA1H and DRVA1L are the same. The same is true forthe signals DRVA2H and DRVA2L.

With the circuit of FIG. 6B, of the first PWM signals DRVA1H and DRVA1L,as the upper arm side signal DRVA1H, the polarity signal Sp is used asis. Also, of the second PWM signals DRVA2H and DRVA2L, as the upper armside signal DRVA2H, a reversed signal of the polarity signal Sp is used.It is possible to suitably control the full bridge circuit 220 whenusing the constitution of either one of FIGS. 6A and 6B.

As can be understood from FIGS. 6A and 6B, with the PWM control unit210, the waveform signal Swave adjusted by the analog waveformadjustment unit 240 is used, so it is possible to execute PWM control ona signal Swave which has a desirable waveform. In particular, theexcitation interval setting unit 245 of the analog waveform adjustmentunit 240 has the special feature of functioning so as to keep the signalSwave at L level near the zero cross point of the sensor output SSA.Near the zero cross point of the sensor output SSA, even when drivevoltage is applied to the coil, a valid drive force is not achieved, andthis causes vibration and noise. Therefore, by adjusting the waveformusing the excitation interval setting unit 245, it is possible toincrease the motor efficiency.

FIG. 7 is a block diagram showing the internal constitution of the fullbridge circuit 220. The full bridge circuit 220 includes four switchingtransistors 221 to 224. The PWM signal DRVA1H, DRVA2L, DRVA2H, andDRVA2L described above are input to the control terminals of theseswitching transistors 221 to 224. Note that it is also possible toprovide a level shifter circuit in front of the control terminals of theupper arm transistors 221 and 223, so as to adjust the level of thedrive signals DRVA1H and DRVA2H.

FIGS. 8A and 8B show an example of the pulse width adjustment with thedrive control circuit of this embodiment. FIG. 8A shows the normalstate. With this normal state, adjustment is not performed by theexcitation interval setting unit 245, and the signal Sd after theoffset/gain adjustment (or the full wave rectification signal Sb) isused as is as the adjusted waveform signal Swave. The original PWMsignal Spwm is a square waveform pulse signal which simulates thevoltage level change of the adjusted waveform signal Swave; so theoriginal PWM signal Spwm is a signal for which pulses are generated withalmost all the sections. FIG. 8B shows an example where the pulse widthof the original PWM signal Spwm is adjusted by changing the offsetadjustment and the gain adjustment from the normal state of FIG. 8A. Asshown with this example, it is possible to adjust the pulse width of theoriginal PWM signal Spwm and thus to control the operation of the motorby changing at least either one of the offset adjustment and the gainadjustment.

FIGS. 9A and 9B show another example of the pulse width adjustment withthe drive control circuit of this embodiment. FIG. 9A shows the samenormal state as FIG. 8A. FIG. 9B shows an example where the pulse widthof the original PWM signal Spwm is adjusted by changing the sawtoothwave signal Ssaw from the normal state of FIG. 9A. As shown with thisexample, it is possible to adjust the pulse width of the original PWMsignal Spwm and thus to control the operation of the motor by changingthe waveform of the sawtooth wave signal.

With the embodiment noted above, the PWM signals DRVA1 and DRVA2 aregenerated from the sensor output SSA using an analog circuit, so it ispossible to generate the PWM signal with a simpler constitution thanwhen using a digital circuit. Also, the sensor output SSA underwent fullwave rectification with the analog waveform adjustment unit 240, and PWMcontrol is executed using the signal after the full wave rectification,so compared to when executing PWM control using a signal which has bothpositive and negative waveform sections, it is possible to simplify thecircuit configuration. Furthermore, the analog waveform adjustment unit240 includes the excitation interval setting unit 245 (valid signalinterval setting unit) that sets only part of the signal after full waverectification as valid, and sets the other part as invalid, so it ispossible to increase motor efficiency.

B. Variation Examples

Note that this invention is not limited to the embodiments andimplementation modes noted above, and it is possible to implementvarious modes in a scope that does not stray from the key points, withthe following variations being possible, for example.

B1. Variation Example 1

With the embodiment noted above, the sawtooth wave signal Ssaw is usedas the PWM control carrier signal, but it is also possible to useanother signal such as a triangle wave or the like as the carriersignal.

B2. Variation Example 2

With the embodiment noted above, a single phase brushless motor is usedas the motor, but the present invention may be applied to other variousmotors. It is also possible to use any value for the motor pole numberand the phase count.

B3. Variation Example 3

With the embodiment noted above, a motor is used as the device to becontrolled which is controlled by the PWM control circuit, but thepresent invention may be applied to circuits for controlling devices tobe controlled other than a motor.

B4. Variation Example 4

The present invention are applicable to various devices and apparatuses.For example, the present invention is applicable to motors and devicesof various kinds such as fan motors, clocks for driving the clock hands,drum type washing machines with single rotation, jet coasters, andvibrating motors. Where the present invention is implemented in a fanmotor, the various advantages mentioned previously (low powerconsumption, low vibration, low noise, minimal rotation irregularities,low heat emission, and long life) will be particularly notable. Such fanmotors may be employed, for example, as fan motors for digital displaydevices, vehicle on-board devices, fuel cell equipped apparatuses suchas fuel cell equipped personal computers, fuel cell equipped digitalcameras, fuel cell equipped video cameras and fuel cell equipped mobilephones, projectors, and various other devices. The motor of the presentinvention may also be utilized as a motor for various types of householdelectric appliances and electronic devices. For example, a motor inaccordance with the present invention may be employed as a spindle motorin an optical storage device, magnetic storage device, and polygonmirror drive. Motors in accordance with the present invention may bealso employed in a moving body and a robot.

FIG. 10 illustrates a projector utilizing a motor according to thepresent invention. The projector 600 includes three light sources 610R,610G, 610B for emitting three colored lights of red, green and blue,three liquid crystal light valves 640R, 640G, 640B for modulating thethree colored lights, a cross dichroic prism 650 for combining themodulated three colored lights, a projection lens system 660 forprojecting the combined colored light toward a screen SC, a cooling fan670 for cooling the interior of the projector, and a controller 680 forcontrolling the overall projector 600. Various rotation type brushlessmotors described above can be used as the motor for driving the coolingfan 670.

FIGS. 11A-11C illustrate a mobile phone utilizing a motor according tothe present invention. FIG. 11A shows the external view of a mobilephone 700, and FIG. 11B shows its exemplary internal configuration. Themobile phone 700 includes a MPU 710 for controlling the operation of themobile phone 700, a fan 720, and a fuel cell 730. The fuel cell 730supplies power to the MPU 710 and the fan 720. The fan 720 is installedin order to introduce air into the interior of the mobile phone 700 tosupply the air to the fuel cell 730, or to exhaust the interior of themobile phone 700 of water which will be produced by the fuel cell 730.The fan 720 may be installed over the MPU 710, as illustrated in FIG.11C, to cool the MPU 710. Various rotation type brushless motorsdescribed above can be used as the motor for driving the fan 720.

FIG. 12 illustrates an electric bicycle (electric-assisted bicycle) asan example of a moving body utilizing a motor according to the presentinvention. The bicycle 800 includes a motor 810 at the front wheel, anda control circuit 820 and a rechargeable battery 830 both attached onthe frame under the saddle. The motor 810 powered by the battery 830drives the front wheel to assist the run. During braking, theregenerated power by the motor 810 is charged in the battery 830. Thecontrol circuit 820 controls the drive and regeneration of the motor810. Various brushless motors described above can be used as the motor810.

FIG. 13 illustrates a robot utilizing a motor according to the presentinvention. The robot 900 includes a first arm 910, a second arm 920, anda motor 930. The motor 930 is used to horizontally rotate the second arm920 as a driven member for the motor. Various brushless motors describedabove can be used as the motor 930.

1. A PWM control circuit that generates PWM signals based on an analogsensor output from a sensor provided in a device to be controlled,comprising: a polarity determination unit that judges positive polaritysections and negative polarity sections of the analog sensor output togenerate a polarity signal; a full wave rectification unit thatgenerates a full wave rectification signal by doing full rectificationof the analog sensor output; an adjustment unit that generates anadjusted waveform signal by adjusting waveform of the full waverectification signal; and a PWM waveform shaping unit that generates afirst PWM signal for the positive polarity section and a second PWMsignal for the negative polarity section, based on the polarity signaland the adjusted waveform signal, wherein the adjustment unit includes avalidity signal interval setting unit that sets only part of the fullwave rectification signal as valid and the other part as invalid.
 2. ThePWM control circuit in accordance with claim 1, wherein the PWM waveformshaping unit includes: a carrier signal generating unit that generates afixed frequency carrier signal; and a comparator that generates anoriginal PWM signal by comparing the adjusted waveform signal and thecarrier signal.
 3. The PWM control circuit in accordance with claim 1,wherein when a position at which the polarity of the analog sensoroutput is reversed is defined as a π phase point, the valid signalinterval setting unit sets as valid the full wave rectification signalin a symmetrical valid section whose center is at a π/2 phase point, andsets as invalid the full wave rectification signal in a symmetricalinvalid section whose center is at the π phase point.
 4. The PWM controlcircuit in accordance with claim 1, wherein the adjustment unitincludes: an offset/gain adjustment unit that adjusts offset and gain ofthe full wave rectification signal.
 5. A method of generating PWMsignals based on an analog sensor output from a sensor provided in adevice to be controlled, comprising: (a) judging positive polaritysections and negative polarity sections of the analog sensor output togenerate a polarity signal; (b) generating a full wave rectificationsignal by doing full rectification of the analog sensor output; (c)generating an adjusted waveform signal by adjusting waveform of the fullwave rectification signal; and (d) generating a first PWM signal for thepositive polarity section and a second PWM signal for the negativepolarity section, based on the polarity signal and the adjusted waveformsignal, wherein the step (c) includes a step of setting only part of thefull wave rectification signal as valid and the other part as invalid.6. The method according to claim 5, wherein the step (d) includes:generating a fixed frequency carrier signal; and generating an originalPWM signal by comparing the adjusted waveform signal and the carriersignal.